The present invention relates to an analog signal generator, and is applied particularly to a digital/analog converter circuit, an AGC (Automatic Gain Controller) and the like.
Heretofore, a variable AMP (AMPlifier) circuit and a tuning circuit have respectively been equipped with a variable capacitive element, so-called varicap and make use of a combined circuit of a DAC (Digital-to-Analog Converter) circuit and an analog filter, and a combined circuit of a PWM (Pulse-Width Modulation) circuit and an analog filter. The varicap is supplied with a bias control signal. In the above built-in combinational circuits, the use of either of the DAC circuit and the PWM circuit has been selected depending on used applications.
When it is desired to cause a variation in bias level to converge earlier and reduce the influence of noise on a peripheral circuit, the DAC circuit has been selected. When the variation in the bias level may be slow or it is not necessary to strictly consider the influence of the noise on the peripheral circuit, the PWM circuit has been selected.
In the case of the former condition, however, the choice of the DAC circuit rather than the choice of the PWM circuit is most suitable as described above. When, however, the DAC circuit is mounted inside a chip of an IC (Integrated Circuit), i.e., a semiconductor element, the DAC circuit entails a demerit in that it is large in chip area as compared with the PWM circuit and current consumption becomes great.
In the case of the latter condition, it is feared that when it is desired to cause the variation in bias level to converge earlier, i.e., shorten the settling time using the conventional PWM circuit, noise will influence the peripheral circuit in the conventional PWM circuit. This is because in view of the mechanism of the PWM circuit, noise is considered to be caused by harmonic components contained in each pulse. The more the speed of each pulse increases, the more there is a fear of the influence of noise thereon. When the conventional PWM circuit is used in this way, the PWM circuit needs to take into consideration a trade-off between relaxation of the influence of noise on the peripheral circuit and the shortening of the settling time at which a desired voltage level is reached.